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Identification

Didemnum vexillum (D. vexillum) is a marine colonial tunicate. Tunicates, in general, are
commonly referred to as ascidians or sea squirts. D. vexillum is known as “carpet tunicate” because of its growth
pattern, or “glove leather tunicate” due to the fact that it feels like soft, wet leather. In Alaska, it is mostly
referred to by its shortened scientific name, “D. vex”; an apt nickname since it reflects the vexing way the
species has been observed taking over marine ecosystems when introduced to new environments.

This colonial tunicate is an invasive fouling species with rapid growth and mat-forming capabilities that colonizes
and dominates artificial and natural hard substrata. It occurs in a wide range of marine habitats worldwide with
potential significant conservation and economic consequences through the alteration of marine habitats and impacts
to mariculture and fisheries, especially shellfisheries.

D. vexillum is difficult to identify correctly with the naked eye because it looks like other didemnids,
some of which are native to coastal Alaska. It may be orange, pink, yellow, tan or cream in color. Its surface
has visible veins with small pores. The texture of the sea squirt is smooth, like firm gelatin and leathery
rather than slimy. The colony may cling strongly to the surface on which it grows and can be difficult to pull
off nets, lines and other fibrous materials. Mature colonies can form large, pendulous lobes that resemble
tendrils of dripping wax when growing on vertical substrate in the water column or in areas with low current
velocity. When growing on the sea floor and in areas with high current velocity it forms low, undulating mats
with short lobes.

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Biology

A colonial tunicate, D. vexillum colonies consist of many small individuals called zooids, each about
a millimeter in length, sharing a common tunic or matrix. Zooids bodies are small, soft, and sac-like.
Tunicates are filter feeders that survive by eating detritus and plankton-tiny animals and plants that drift
in the ocean. Each zooid pumps water through its body, extracting food particles from the water and then,
along with neighboring zooids, discharges the filtered water into a common space. The water leaves the colony
from shared siphons. Siphons appear as small pores in the surface of the tunicate. The siphons are clearly
visible on solitary tunicates, and sometimes expel water when gently squeezed, thus the name “sea squirt”.

In the field, tunicates can be difficult to distinguish from sponges. Sponges tend to feel “spongy”- springing
back when compressed, tunicates on the other hand resist compression. Rubbing a sponge animal between the
fingertips feels gritty rather than slippery. Embedded within the thin layer of the tunic, though not clearly
visible to the naked eye, are tiny, spiky balls comprised of calcium. The presence of these barbed balls can
help differentiate between various colonial tunicates.

Colonial ascidians are hermaphroditic- they are both male and female at the same time- and reproduce sexually
by broadcast spawning, and asexually by budding. D. vexillum broods larvae in the colony’s matrix.
The larvae spend a few hours moving within the water current before settling on a firm surface. The larvae
metamorphose into zooids that can then start a new colony. Larval settlement happens at different times
depending on a number of habitat parameters, such as water temperature. When lobes, sections of tunicate
colonies, break off, they can drift in the water to a new site, resettle on hard substrate, and begin a
new colony.

Because it can grow rapidly when conditions are right, D. vex fouls and overgrows structures, benthic
organisms and shellfish. Mariculture (marine aquatic farming) gear, docks, and buoys can be overgrown
rapidly, requiring aquatic farmers to de-foul their gear more often to provide adequate filtration for
shellfish growth, potentially effecting the mariculture industry. Natural environments, from intertidal
and nearshore areas to offshore fishing grounds, have been infested with the carpet tunicates. Substantial
impacts on marine food webs may result when invasive tunicates smother organisms living on the sea bottom,
thereby concealing food from predators.